There have been proposed an image forming apparatus, in which transfer rollers are provided to face photosensitive drums, and a transfer belt is interposed between the transfer rollers and the photosensitive drums. In such an image forming apparatus, a rotation center of each of transfer rollers is provided to deviate from a corresponding photosensitive drum toward the downstream side of the corresponding photosensitive drum in a movement direction of an intermediate transfer belt. For example, as shown in FIG. 10, a rotation axis line Z of a transfer roller 100 is provided to deviate from a rotation axis line X of a photosensitive drum 101 toward the downstream side of the rotation axis line X in the movement direction of the intermediate transfer belt 102, and to be pushed up.
If the transfer roller 100 is provided as described above, in each transfer unit where a toner image is transferred onto the intermediate transfer belt 102, as shown in FIG. 10, on the intermediate transfer belt 102, an upstream nip H, a physical nip I, and a downstream nip J are formed.
The physical nip is an area on the intermediate transfer belt 102 where the intermediate transfer belt 102 is put between the photosensitive drum 101 and the transfer roller 100. A toner image formed on the photosensitive drum is transferred onto the intermediate transfer belt 102 at the physical nip I.
The upstream nip H is located upstream of the physical nip I in the movement direction of the intermediate transfer belt 102. The upstream nip H is an area on the intermediate transfer belt 102, where only the photosensitive drum 11 comes into contact with the intermediate transfer belt 102. Before coming into contact with the transfer roller 100, the intermediate transfer belt 102 conies into with the photosensitive drum 101 at the upstream nip H.
The downstream nip J is located downstream of the physical nip I in the movement direction of the intermediate transfer belt 102. The downstream nip J is an area on the intermediate transfer belt 102 where only the transfer roller 100 and the intermediate transfer belt 102 come into contact with each other. At the downstream nip J, even if the intermediate transfer belt 102 is separated from the photosensitive drum 101, the intermediate transfer belt 102 comes into contact with the transfer roller 100.
If the upstream nip the physical nip I, and the downstream nip J are secured as described above, good toner-image transfer becomes possible. Specifically, the following reasons may be considered. Before a toner image on the photosensitive drum 101 reaches the physical nip I, at the upstream nip H, the toner image is nipped by the photosensitive drum 101 and the intermediate transfer belt 102. Therefore, the toner image on the photosensitive drum 101 may fly from the photosensitive drum 101 before reaching the physical nip I, such that a smaller amount of toner is transferred onto the intermediate transfer belt 102 (pre-transfer occurs).
Further, since the physical nip I is provided, an electric field sufficient for transfer is formed between the photosensitive drum 101 and the transfer roller 100. Therefore, at the physical nip I, the toner image on the photosensitive drum 101 is transferred onto the intermediate transfer belt 102.
Further, at the downstream nip electrical charge having moved to the intermediate transfer belt 102 at the upstream nip H or the like returns o the transfer roller 100. Therefore, it is possible to reduce peeling discharge when the photosensitive drum 101 and the intermediate transfer belt 102 are separated from each other.